1. Field of the Invention
The present invention relates to the field of accelerometers and more particularly to the field of optical accelerometers or laser accelerometers having a pendulous mass that is force rebalanced or restored in response to an acceleration sensed on the accelerometer's input axis.
2. Description of The Prior Art
Electronically rebalanced or restored accelerometers that rely on Hooke's Law are known.
A laser accelerometer is shown in U.S. Pat. No. 4,048,859 to Gary D. Babcock and entitled "Laser Accelerometer" and assigned to Rockwell International Corporation. The Babcock accelerometer is a single axis accelerometer that does not utilize an electronically restored or rebalanced pendulous mass. In the Babcock instrument, means are provided to assure that two orthogonally polarized modes are excited in a gas laser cavity. A birefringent element contained in the cavity is traversed by light in both modes. A proof mass attached to the birefringent element applies stress to it in response to acceleration along one particular axis. The material index of refraction change produced by the acceleration-induced stress is different for the two orthogonally polarized lasing modes, resulting in a change in the frequency difference between them. This frequency difference is measured and translated into sensed acceleration. The Babcock instrument is not a clear path instrument. In the Babcock instrument, the sensing element is inserted into the laser cavity and positioned in the laser beam where the element produces a cavity loss which results in increased energy consumption lessening the efficiency and accuracy of the instrument.
U.S. Pat. No. 4,637,255 issued Jan. 20, 1987 to Graham J. Martin and assigned to Litton Systems Inc. shows an out of plane ring laser gyro cavity having a proof mass positioned atop the instrument. The cavity in the housing accommodates a laser gain medium. Mirrors are used within the cavity to direct light in a non-planar ring around the cavity. As the proof mass is acted on by acceleration along a preselected sensitive axis, it transfers a force to the cavity structure thereby slightly deforming the cavity. Deformation of the cavity in response to an acceleration input results in a change in the frequency splitting between the two modes of different polarization within the cavity. The frequency splitting is measured and related to sensed acceleration.
The Martin instrument is a clear path instrument. The Martin instrument is a single axis instrument and cannot be adapted to sense acceleration on three separate and orthogonal axes.
Use of synchronous demodulation in connection with extracting signal information from the Martin and Babcock instruments is not taught or suggested in either reference.